Pyrolysis of hydrocarbons



Patented Nov. 16, 1931 PATENT OFFICE PYROLYSIS F HYDROCARBONS Wesley C.Stoesser, Midland, Mich; assignor to The Dow Chemical Company, Midland,Mich, a corporation of Michigan No Drawing. Application May 1'1, 1935,

Serial N0. 22,105

Claims.

One of the difllculties commonly associated with processes in whichhydrocarbons are heated, either in gas or liquid phase, at hightemperatures is the formation of carbon on the heating surfaces. Suchcarbon formation results in a loss'of materials in process, while thecarbon deposits so formed interfere with the transfer of heat andfrequently cause interruptions of the process 'for removing carbonaccumulations which obstruct the apparatus.

In apparatus of the type used for pyrolyzing or cracking hydrocarbonsiron or steel is the usual material of construction wherever it can beemployed without detriment. It is known,

20 however, that at the temperatures used in various cracking processese. g. 500 to 900 0., iron promotes the formation of solid carbon,apparently acting as a catalyst for breaking down the hydrocarbonmolecules to elementary car- 5 born. Various expedients have beenproposed for avoiding the effects of the aforesaid catalytic action ofiron in contact with hydrocarbons at cracking temperatures. Forinstance, it has been proposed to-use special steels of high chromium 0content, but such steels are more costly than ordinary carbon steels,andin a large installation their use adds greatly to the capital costand increases correspondingly the cost of the prod not. It is desirableto make possible and pracggtlcable the use of usual carbon steels inthe' construction of apparatus for the pyrolysis of hydrocarbons.

Particularly in connection with the pyrolysis of benzene and otheraromatic hydrocarbons for n the formation of diphenyl and related diarylcompounds it has been found that, when the pyrolysis is carried out iniron apparatus, much carbon is formed, which deposits on the walls ofthe apparatus, sometimes in hard compact form, at

45 others in loose form, depending upon variations in operatingconditions. These carbon deposits greatly interfere with the smooth andcontinuous operation of the process, so much so that it has generallybeen regarded as impracticable to 50 use tubular apparatus of wroughtiron or carbon steel for pyrolyzing benzene hydrocarbons in theusualtemperature range of about 650 to 850 0., because the tubes rapidlybecome filled and stopped with carbon. It has therefore been pro- 55posed to employ special chromium steels for the 1 (c1. zoo-110iapparatus. or to provide special coatings or linings on ordinary steelwhich inhibit-or reduce the formation of carbon, or to introduce alongwith the pyrolysis gases or vapors a foreign substance which has theeffect of counteracting the forma tion of carbon.

I have now found that the substantial pre vention of carbon formation inthe pyrolysis of hydrocarbons, generally, and particularly benzenehydrocarbons, when'carried out in apparatus of iron or. carbon steel,can 'be attained by much simpler means 'than those hitherto used. Theinvention is based upon my discovery that a preliminary treatment of theiron surfaces of the pyrolyzing apparatus with ammonia serves tosuppress the catalytic action of iron in causing formation of carbon inthe apparatus subsequently when hydrocarbons are heated to a pyrolyzingtemperature therein.

It is among the objects of the invention to provide a method ofpyrolyzing hydrocarbons in ordinary iron or steel apparatus wherein theformation of carbon is greatly reduced or even substantially prevented,whereby the process can be carried on continuously for long periods oftime without interruptionv due to carbon accumulation. Another object isto provide a method of treating such iron apparatus for pyrolysis ofhydrocarbons, particularly benzene hydrocarbons, so that carbonformation is practically inhibited therein during such pyrolysis.Another object is to enable the use of wrought iron or common mildsteels as structural material of apparatus for carrying out theaforementioned pyrolytic reactions. Other objects will appear from thefollowing specification in which the invention is fully described andparticularly pointed out in the claims. I

For purpose of illustration the invention will be described withparticular reference to the 40 pyrolysis of benzene and benzenehydrocarbons for the production of diphenyl and relateddi arylcompounds, but in its broad scope the invention is applicable toprocesses for the .pyrolysis of hydrocarbons in general, especially ingas or vapor phase, such as in the cracking of petroleum hydrocarbonsfor the production of motor fuel or of gaseous oleflnes, and the like.

During a series of experiments in the pyrolysis of benzene vapors at atemperature in the range of 650 to 850 C., in which the vapors werepassed through heated tubes of wrought iron or low carbon steel, it wasfound that within a few hours at most the tubes would become practicallyblocked with carbon. This result occurred ir- 55 respective of thepurity of the benzene, or of the rate of vapor flow, the carbonformation of course being the more rapid the higher the temperature.This situation indicated the practical impossibility of using ordinaryiron or steel tubes in a commercial apparatus for making diphenyl anddiaryl compounds by pyrolyzing benzene hydrocarbons, in which processthe operation would have to be practically continuous over aconsiderable period of time in order to be commercially economical.However, I found that, if such tubes were subjected to the action ofammonia at a moderately elevated temperature for a short time prior toemploying them for the pyrolysis of benzene, the latter process couldthen be carried out in the tubular apparatus uninterruptedly for periodsof several days, even weeks, without any substantial formation ofcarbon.

The treatment with ammonia may easily be carried out in situ with theapparatus completely assembled for use in the pyrolytic process. Thefurnace section of the apparatus in which cracking is to be carried outis simply heated so that the cracking tubes, or the equivalent thereof,are heated to a temperature approximately in the range of 300 to 600 C.,and ammonia is passed through the apparatus for a short time, e. g. fromabout to 2 hours, but not necessarily exceeding a few hours at most. Forexample, at a temperature ofaround 500 C., a period of from 30 minutesto 1 hour is sufficient for the ammonia treat- 'ment. At lowertemperatures a somewhat longer time may be requi e to give the bestresults. After such preliminary treatment with ammonia the apparatus maybe heated to the regular operating temperature and the hydrocarbonvapors passed therethrough, the pyrolyzing process then being continuedindefinitely without substantial deposition of carbon, whereas withoutsuch ammonia treatment the tubes would be blocked with carbon within afew hours at most.

The effect of the aforesaid ammonia treatment in causing suppression ofcarbon formation in iron apparatus is not completely understood. AnX-ray examination has shown that the ammonia treatment produces a thinfilm of iron nitride on the exposed surface of the iron. However,subsequent exposure of such nitride coating to hydrocarbon vapors for ashort time at the pyrolyzing temperature results in the disappearance ofthe iron nitride film, at least to the extent that it is no longerdetectable by x-ray test. Nevertheless, the disappearance of the nitridefilm does not affect the ability of the iron surface to resist orprevent the formation and deposition of carbon thereon, in view of thefact that such a surface has been employed continuously for many days inthe pyrolysis of benzene without formation of carbon taking placethereon. Apparently the action of ammonia on the iron surface renders itpassive with respect to the normal property of iron to catalyze carbonformation under the conditions existing in pyrolysis reactions.

As a specific example of the practice of the invention, benzene wasvaporized at therate of 2 cubic centimeters per minute and the vaporswere passed through a standard 1 inch iron pipe 53 inches long heated ina furnace over a length of 47 inches, so that the maximum temperature atabout the middle of the pipe was '760-'I90 C. Under these conditions thepercentage conversion of benzene to diphenyl and higher diaryls wasapproximately 8 per cent. In 11.5 hours the pipe was completely stoppedwith carbon. The pipe was removed and another piece of the same leng hinserted in the furnace, and ammonia gas was passed through the pipe forhour. Thereafter benzene vapors were passed through the pipe at the samerate as before, and at an average maximum temperature of 737 C.continuously for 89.5 hours, giving a conversion of 9.23 per cent, atthe end of which time no perceptible deposit of carbon had formed in thepipe. The product of the pyrolysis consisted of 86.7 per cent ofdiphenyl, and 13.3 per cent of higher boiling compounds.

In. other experiments conducted similarly in iron pipes at temperaturesvarying from about 700 to 760 C. maximum, and at varying rates of vaporflow corresponding to the vaporization of from 2 to 11 cubic centimetersof benzene per minute, the pipes were stopped with carbon in from 3 to 5hours. When similar iron pipes were given a preliminary treatment withammonia at about 400 to 500 C. for about 1 hour, and then used topyrolyze benzene under similar conditions of vapor flow and temperaturethey were run as long as 200 hours continuously without materialformation of carbon.

By preventing carbon formation, the invention has the further advantagethat a higher conversion of benzene to diphenyl is obtained, otherconditions remaining the same. In fact, as shown in the foregoingexample, a higher conversion can be obtained at a somewhat lowertemperature. The invention therefore not only overcomes the mechanicaland operating diihculties previously experienced, thereby makingpossible by simpler means than hitherto the commercial adaptation ofiron apparatus in the pyrolysis of benzene and other hydrocarbons, butit also results in an increased yield of product and materially raisesthe efficiency of the process.

when treating the iron tubes, or other ferrous apparatus, with ammoniain accordance with the invention, it is convenient, but not necessary,to maintain a slight gas pressure, e. g. 5 to 10 pounds gauge, in theapparatus. The ammonia treatment may be carried out at any temperaturesuitable for nitridlzing iron, e. g. between about 300 and 700 0.,preferably between about 450 and 550 0.

Similarly, ferrous reactors used for the pyrolysis of otherhydrocarbons, such as petroleum and fractions thereof, and aliphatic oraromatic hydrocarbons generally, may be given a preliminary treatmentwith ammonia to suppress the formation of carbon therein subsequentlyduring the pyrolysis of the hydrocarbons. The treatment may beconveniently carried out in situ with the apparatus already set up foroperation, to coat the interior walls, at least initially, with a filmof iron nitride, or the parts thereof may be separately treated withammonia in any suitable way and later assembled for use in the process.

Other modes of applying the principle of my invention may be employedinstead of those explained,'change being made as regards the methodherein disclosed, provided the step or steps stated by any of thefollowing claims or the equivalent of such stated step or steps beemployed.

I therefore particularly point out and distinctly claim as myinvention 1. In a method of pyrolyzing hydrocarbons in apparatusconstructed of iron or steel, the improvement which consists inpreventing or reducing theformation of carbon by passing the hydrocarbonvapors at a pyrolyzing temperature through such apparatus of iron orcarbon steel,

. thereon.

2. In a method of pyrolyzing benzene hydrocarbons in apparatusconstructed of iron or steel, the improvement which consists inpreventing or reducing the formation of carbon by passing the benzenehydrocarbon vapors at a pyrolyzing temperature through such apparatus ofiron or carbon steel, the walls of which exposed to contact with thevapors have been previously subjected to the action of ammonia at anitridizing temperature for a time sumcient to form a nitride coatingthereon.

3. In a method of pyrolyzing benzene hydrocarbons in apparatusconstructed of iron or steel, the improvement which consists inpreventing or reducing the formation of carbon by passing the benzenehydrocarbon vapors at a pyrolyzing tem- .perature through such apparatusof iron or carbon steel, the walls of which exposed to contact with thevapors have been previously subjected to the action of ammonia at atemperature between about 300 and about 700 C. for a time sufflcient toform a nitride coating thereon.

4. In a method of pyrolyzing benzene hydrocarbons in apparatusconstructed of iron or steel, the improvement which consists inpreventing or reducing the formation of carbon by passing the benzenehydrocarbon vapors at a pyrolyzing temperature through such apparatus ofiron or carbon steel, the walls oi which exposed to contact with thevapors have been previously subjected to the action of ammonia at atemperature between about 450" and 550 C. for a time sufllcient to forma nitride coating thereon.

5. In a method of pyrolyzing benzene hydrocarbons in apparatusconstructed of iron or steel, the improvement which consists inpreventing or reducing the formation of carbon by passing the benzenehydrocarbon vapors at a pyrolyzing temperature through such apparatus ofiron or carbon steel, the walls of which exposed to contact with thevapors have been previously subjected to the action of ammonia at atemperature between about 450 and 550 C. for a period of from aboutone-half hour to about two hours.v

- WESLEY C. STOESSER.

